The present invention relates generally to digital modulated signal compressing and expanding systems, and more particularly to a system in which the transmission of a control signal which indicates the information concerning the compression level of the signal upon expanding the level of the digital modulated signal at the receiving end, which is transmitted from the transmitting end after being compressed of its level, is not required, and, furthermore capable of transmitting the above signal by use of few number of bits.
Conventionally, one example of a digital modulated signal compressing and expanding system were of the following construction. That is, an analog signal is supplied to a low-pass filter wherein the unessential high-frequency component of the signal is eliminated, and this filtered signal is supplied to a sampling and holding circuit wherein the signal is sampled and held. The discrete sampled signal obtained from the sampling and holding circuit, is applied to an A/D (Analog-To-Digital) converter through a variable gain circuit. On the other hand, this discrete sampled signal is also supplied to an absolute-value circuit, wherein the absolute value is detected. The output signal of this absolute-value circuit, is supplied to a voltage comparator wherein the output signal thus supplied to the voltage comparator, is compared with a reference voltage supplied from reference voltage setting circuit. When the above output signal which is compared with the reference voltage exceeds the reference voltage, the gain of the above variable gain circuit is varied.
When the absolute value of the input voltage of the variable gain circuit becomes large, and becomes a voltage which is the minimum or the maximum voltage which is possible to be transmitted by the A/D converter, the variable gain circuit is varied of its gain by a gain control signal supplied from the above voltage comparator. As a result, the absolute value of the above output voltage is attenuated. Accordingly, the output voltage of the variable gain circuit, is always within a predetermined voltage range which is determined by the number of bits in the A/D converter.
The output voltage of the above described variable gain circuit is supplied to the A/D converter, wherein the output voltage undergoes analog-to-digital conversion (to be concrete, the output voltage is quantized and encoded) and converted into a digital modulated signal such as a pulse code modulation (PCM) signal. This digital modulated signal is supplied to a D/A (Digital-to-Analog) converter through a predetermined transmission path. The digital modulated signal thus supplied to the D/A converter, undergoes digital-to-analog conversion therein, and by use of another variable gain circuit, the wave portion which were attenuated or amplified of its level, is returned to its original state by the level expansion performed in the transmission system.
Therefore, in the above described conventional digital modulated signal compressing and expanding system, a variable gain circuit control signal which controls the gain of the variable gain circuit at the receiving end by the same amount the variable gain circuit at the transmission end was gain-controlled, in an opposite direction the variable gain circuit at the transmission end was gain-controlled, must also be transmitted as a transmission signal. Accordingly, there was a limit in reducing the number of transmission bits.
Generally, in the transmission system or the recording system for digital modulated signals, the cost and the complexity in the construction of the apparatus, is greatly affected by the capacity of signals which are to be processed. Hence, it is highly desirable to reduce the signal capacity as much as possible, within the range which maintains the allowable transmission quality.